Method and arrangement at a loading column

ABSTRACT

A method and arrangement for reducing the evaporation of volatile organic compounds (VOC) or other gases during the filling of an essentially liquid petroleum product on a storage and/or transport tank ( 2 ) via a feed pipe ( 6 ), and where the petroleum product is led into the storage/transport tank via a loading column ( 8 ) having a significantly larger cross section than that of the feed pipe ( 6 ).

This invention regards a method of reducing that proportion of volatileorganic compounds (VOC) which is separated from oil, in particular crudeoil, during the filling of tanks, such as during the loading of a shipfrom a terminal, a production platform or a floating loading device. Theinvention also comprises an arrangement for carrying out the method. Thearrangement may also be used for reducing evaporation of gas during thefilling of single components such as propane, butane, ethane andliquefied natural gas.

Crude oil is made up of different components stabilised at a specifiedpressure and a specified temperature. If these conditions are altered,either through a reduction in pressure or an increase in temperature, aproportion of the volatile components will separate out and gasify.These components consist of volatile organic compounds such as e.g.methane, propane, butane and ethane, and are termed VOC. Systemscurrently exist for removal of these gases. As opposed to thearrangement of the present application, today's systems are based ontreating the already separated gases by providing a process arrangementfor treatment of the exhaust gas subsequent to evaporation. The plantsare complex and will require a lot of energy, as pressure andtemperature are employed to bring the gases back to a liquid state.

It is common knowledge that a relatively large quantity of volatileorganic compounds evaporate during pumping of oil into large tanks.Under normal circumstances, a pressure of the order of 1.05 to 1.07 baris maintained both in storage and transport tanks. When loading e.g. atanker, it is customary for the oil to be pumped from a storage tankthrough a feed pipe to a position above the cargo tank, from where theoil is sent into the tank through a drop line (downcomer) to the bottomof the tank. A drop line of this type may have a length of the order ofseveral tens of metres.

When the oil flows into the upper end portion of the drop line, gravitywill accelerate the liquid flowing down through the drop line, whereby alower static pressure is created in the feed pipe and the upper portionof the drop line. In these pipes, where the static pressure is lowerthan the vapour pressure, the evaporation of volatile organic compoundsis significant, and these compounds will only to a small extent condenseback to a liquid state upon resumption of normal tank pressure.

The object of the invention is to remedy the disadvantages of prior art.

The object is achieved in accordance with the invention by thecharacteristics stated in the description below and in the appendedclaims.

Experiments have shown that if the incoming fluid is passed into anadapted hollow column positioned in or in connection with thestorage/transport tank, the evaporation of gas from the fluid is reducedto a considerable extent.

The hollow column is formed as a vertical loading column, preferablywith a tangential inlet near the upper end portion and an outlet nearthe lower end portion. The outlet discharges into the bottom or thepiping system of the storage/transport tank and will, after inflow offluid, be submerged in the contents of the storage/transport tankwithout subjecting the fluid to any significant underpressure.

As the inflowing fluid drops from the inlet at the upper portion of theloading column to the bottom of the loading column, or possibly to alevel corresponding to the fluid level in the storage/transport tank, aninitial evaporation of gasses from the inflowing fluid takes place. In aloading column of the applied-for type, the inflowing fluid is notsubjected to a drop in static pressure similar to that experienced whenflowing down a drop line, as is common when using prior art. After arelatively short inflow period, the atmosphere in the loading column issaturated with gas that has evaporated from the inflowing fluid, afterwhich any further evaporation will be insignificant.

The so-called Froude number is known from the theory of the hydraulicsof open channels. The Froude number B, which is dimensionless, isdefined as a ratio between the force of inertia and the force of gravityacting on a fluid:F=V/{square root}{square root over (gh _(m) )}where V=fluid velocity in metres per second, g=the earth's gravity inmetres per second² and h_(m)=the hydraulic mean depth.

By replacing the hydraulic depth h_(m) in the formula with the diameterD of the loading column, an expression is found which has proven to beexpedient for selecting a suitable loading column diameter.

The development work carried out has shown that the evaporation isreduced when the value of the expressionF=V/{square root}{square root over (gD)}is less than 0.45. At 0.31, the pressure in the column will be balanced.The best effect is achieved at values of less than 0.18.

Thus the diameter of the loading column is principally dependent on thevelocity of the inflowing fluid.

Advantageously, the upper portion of the loading column may communicate,preferably via at least one pressure relief valve, with thestorage/transport tank-being filled, or with another tank. Thus anyoverpressure or underpressure in the loading column may be evacuated orequalized by gas transport between the loading column and thecorresponding tank.

Advantageously, the outlet of the loading column is designed accordingto known laws of fluid flow in order to ensure a laminar flow, and alsoto ensure that the outlet is submerged by the inflowing fluid after arelatively small amount of fluid has been filled on thestorage/transport tank.

The method and the arrangement according to the invention are wellsuited for use during loading of ships and other large tank farms whendealing with substantially liquid petroleum products.

The following describes a non-limiting example of a preferredarrangement and method illustrated in the accompanying drawing, inwhich:

FIG. 1 schematically shows a section of a loading arrangement in whichoil is pumped on board a tanker equipped with a loading column. A spiralprovided with arrows indicates the flow path of the oil in the loadingcolumn, while ellipses, also provided with arrows, indicate a possibleflow path for gas in the loading column.

In the drawing, reference number 1 denotes a ship with a transport tank2 for oil. When empty, the ship 1 rides relatively high in the water 4.Oil flows from a pumping station (not shown) through a loading pipe 6,tangentially into the upper end portion 10 of a loading column 8. Thecross section of the loading column 8 is significantly larger than thatof the loading pipe 6. The loading pipe 6 may be in the form of e.g. apipe, a hose or another suitable hollow body.

In this preferred embodiment, the loading column 8 constitutes a part ofthe transport tank 2 on board the ship 1 and has been formed as acylindrical silo, the upper end portion 10 of which is equipped with acover 12, and the lower end portion 14 if which is equipped with anoutlet 16 that discharges into the transport tank 2.

The upper portion 10 of the loading column 8 is connected to and incommunication with the transport tank 2 via branch pipes 18 and 20, aswell as valves 22 and 24. The pressure relief valve 22 is designed toopen for flow from the loading column 8 to the cargo hold 2 at apredetermined pressure differential, while the pressure relief valve 24is designed to open for flow from the cargo hold 2 to the loading column8 at a predetermined pressure differential.

Oil containing relatively volatile components is pumped through theloading pipe 6 into the upper end portion 10 of the loading column 8,where, due to the tangential connection of the feed pipe 6 to theloading column 8, it assumes a helical flow pattern down through theloading column 8. In FIG. 1, the flow is illustrated by means of aspiral line with arrows.

The oil then flows out through an opening 16 in the lower end portion 14of the loading column 8, the opening 16 ending up near the bottom of thetransport tank 2. In order to prevent vortices from occurring at theopening 16, a means of preventing this may be installed near the opening16. Advantageously, the opening 16 is designed so as to become submergedafter a relatively small quantity of oil 17 has been pumped into thetransport tank 2.

When pumping the first quantity of oil, some of the more volatilecomponents of the oil evaporate during the flow of the oil through theloading column 8. After a relatively small quantity of oil has beenpumped in, the atmosphere in the loading column becomes saturated withvolatile gases, whereby further evaporation of gases from the oil isessentially stopped.

It is assumed that the gases present in the loading column duringloading are in motion. Ellipses with arrows in FIG. 1 illustrate apossible flow path.

The favourable effect of the invention is not dependent upon the loadingpipe 6 being connected to the loading column 8 in a tangential manner,but experiments show such geometry to be favourable.

1. A method of reducing the evaporation of volatile organic compounds(VOC) or other gases during the filling of an essentially liquidpetroleum product on a storage or transport tank (2) via a feed pipe(6), characterized in that the petroleum product is discharged from thefeed pipe (6) into a loading column (8), the loading column (8) having asignificantly larger cross section than that of the feed pipe (6).
 2. Amethod in accordance with claim 1, characterized in that the petroleumproduct is led tangentially into the loading column (8).
 3. Anarrangement for reducing the evaporation of volatile organic compounds(VOC) during the filling of an essentially liquid petroleum product on astorage or transport tank (2), characterized in that a feed pipe (6)have its discharge opening in a loading columns (8) upper end portion(10), the cross section of the loading column (8) being significantlylarger than that of the feed pipe (6), the loading column (8) having itsdischarge opening (16) at its lower end portion (14).
 4. An arrangementin accordance with claim 3, characterized in that the feed pipe (6) isconnected to the loading column (8) in an essentially tangential manner.5. An arrangement in accordance with claim 3, characterized in that theloading column (8) is placed principally in a transport or storage tank(2).
 6. An arrangement in accordance with claim 3, characterized in thatthe loading column (8) has an opening (16) that discharges near thebottom of the transport or storage tank (2).
 7. An arrangement inaccordance with claim 3, characterized in that the upper end portion(10) of the loading column (8) is connected to the transport or storagetank (2) in a communicating manner.